fix: make sure to set content-type for file type (#3837)

This PR fixes a hidden bug that has been here since some time, where the response headers content-type was not set.

This will failed if the user is using the client, as the fake_req actually will try to get the content-type from the headers.

src/bentoml/_internal/io_descriptors/file.py

@@ -228,7 +228,14 @@ async def to_http_response(self, obj: FileType, ctx: Context | None = None):
             )
             set_cookies(res, ctx.response.cookies)
         else:
-            res = Response(body)
+            res = Response(
+                body,
+                headers={
+                    "content-type": self._mime_type
+                    if self._mime_type
+                    else "application/octet-stream"
+                },
+            )
         return res
 
     async def to_proto(self, obj: FileType) -> pb.File: